Method for estimating a plane in a range image and range image camera

ABSTRACT

3D data of pixels arranged in a square lattice is converted to a houndstooth arrangement, a largest value among respective magnitudes of cross products of all pairings of the normal vector of a small triangle of interest and the normal vectors of three adjacent small triangles is derived, and a large triangle composed of the small triangle of interest and the adjacent small triangles is determined to be a plane candidate if the largest value is less than or equal to a first threshold, after which similar processing is repeated with this large triangle taken as a small triangle, and this repetition is recursively performed while enlarging the small triangle of interest, and when the area of a new large triangle is greater than or equal to a predetermined area, the region thereof is determined to be a plane and separated from a range image.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(a) on PatentApplication No. 2010-050906 filed in Japan on Mar. 8, 2010, the entirecontents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for estimating a plane in acaptured range image and to a range image camera, and more particularlyto a method for estimating a plane in a captured range image byprecisely measuring per pixel the time taken for projected light to bereflected back by a target, and to a range image camera to which such aplane estimation method is applied.

2. Related Art

Heretofore, various techniques that involve using a captured image of atarget space to perform processing such as monitoring the target spaceand extracting the appearance of a target object in the target spacehave been proposed. However, there is a problem in that because theimage used in such techniques is a gray scale image that reflects thetones of the target space and is thus affected by changes in the amountof external light, these techniques can only be used in an environmentin which there is very little change in the amount of light.

In view of this, techniques have also been proposed that enable a targetobject to be extracted without being affected by changes in the amountof light, by employing a configuration for generating a range imagehaving ranges for pixel values, and extracting the target object usingthe range image.

However, with such a range image, the range values may be unstable withonly the data of single pixels, making it difficult to clearly separatebetween a human body serving as the target object to be extracted and afloor surface. Thus, at present, the foot of a human body is separatedfrom a floor surface using devices such as increasing the angle ofelevation, so as to intentionally not include the contact area betweenthe foot of the human body and the floor surface in the viewing angle,for example.

As for image processing apparatuses using range images, techniques havebeen proposed that, for example, enable a target object to be separatedeven in the case of a small range difference between the target objectand the background, by generating a difference image from a plurality ofrange images obtained at different times (e.g., see JP 2006-053791A).

However, these techniques simply separate the background, and do notactively recognize planes in a range image.

As for methods for searching for planes in a captured image, techniquessuch as “Plane Estimation Method using Stereo Images and ObjectDetection Apparatus” (JP 2004-094707(a), hereinafter “Patent Document2”), “Autonomous Mobile Apparatus and Planar Obstacle RecognitionMethod” (JP 2007-121258A, hereinafter “Patent Document 3”), and “RobotApparatus, Stair Climbing Method therefor, and Mobile Apparatus”(WO2005/087452A, hereinafter “Patent Document 4”) have also beenproposed.

Of these, the technique disclosed in Patent Document 2, althoughenabling a plane to be estimated with a high degree of accuracy evenwhen an object other than a plane exists in an imaging area, is limitedto use with stereo images. The main object of the technique disclosed inPatent Document 3 is to accurately recognize the position of a planarobstacle serving as an obstacle, particularly the position of a planarobstacle having a void. The technique disclosed in Patent Document 4 isconfigured such that recognition of a step having a movable plane andjudgment as to whether the step can be climbed are performed, so as toenable the mobile object to perform an autonomous climbing operationitself after acquiring information relating to the step.

In order to animate a human body model freely in a range image with asmall amount of computation, it is necessary to clearly separate thefloor surface and the foot with small amount of computation. However,the abovementioned conventional techniques are not necessarily suitablefor realizing this.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for estimatinga plane in a range image that enables the foot of a human body to beclearly separated from a floor surface with a small amount ofcomputation, even in the case where, for example, a contact pointbetween the foot of the human body and the floor surface is included inthe captured range image, and a range image camera to which such a planeestimation method is applied.

A method for estimating a plane in a range image according to thepresent invention includes a data conversion step of computing convertedthree-dimensional data that is converted to a houndstooth arrangement,based on three-dimensional data of pixels arranged in a square lattice,a triangle-of-interest computation step of focusing on one of smalltriangles constituting a smallest unit of the houndstooth arrangement,and computing, based on converted three-dimensional data correspondingto three vertices of the small triangle of interest, an equation of aplane defined by the three vertices, and a cross product of two vectorscorresponding respectively to two sides of the small triangle ofinterest as a normal vector of the plane, an adjacent trianglecomputation step of computing a normal vector for each of three smalltriangles that are adjacent in such a manner as to respectively share adifferent one of three sides of the small triangle of interest, a planecandidate search step of searching for a plane candidate by performing,for all of the converted three-dimensional data while shifting the smalltriangle focused on in the triangle-of-interest computation step, aprocess of deriving a largest value among respective magnitudes of crossproducts of all pairings of the normal vector computed in thetriangle-of-interest computation step and the three normal vectorscomputed in the adjacent triangle computation step, and a process ofdetermining a large triangle composed of the small triangle of interestand the three adjacent small triangles to be a plane candidate if thelargest value is less than or equal to a first threshold, a planedetermination step of sequentially repeating the triangle-of-interestcomputation step, the adjacent triangle computation step and the planecandidate search step with a large triangle determined to be a planecandidate in the plane candidate search step taken as the small triangleof interest in the triangle-of-interest computation step, recursivelyperforming the repetition while enlarging the small triangle ofinterest, and, when an area of a new large triangle determined to be aplane candidate is greater than or equal to a second threshold,determining a region corresponding to the large triangle to be a plane,and a plane-determined region separation step of separating the regiondetermined to be a plane in the plane determination step from a rangeimage.

According to the method for estimating a plane in a range image havingsuch a configuration, the foot of a human body can be clearly separatedfrom a floor surface with a small amount of computation, even in thecase where, for example, the contact point between the foot of the humanbody and the floor surface is included in the captured range image,detection of a human body, object or the like can be more accuratelyperformed. Further, falsely recognizing a smooth curve or the like as aplane can be prevented to the utmost, and even if a very small stepexists in a surface that can as a whole be taken as a plane, the surfacecan be accurately recognized as being a plane.

Also, in the method for estimating a plane in a range image of thepresent invention, even in a case where a second large triangle that isadjacent in such a manner as to share one side of a first large triangledetermined to be a plane candidate in the plane candidate search step isnot determined to be a plane candidate, if a magnitude of a crossproduct of a normal vector of the first large triangle and a normalvector of each of three small triangles included in the second largetriangle and adjacent to the first large triangle is less than or equalto a third threshold, the small triangles may each be taken as a planecandidate.

According to the method for estimating a plane in a range image havingsuch a configuration, even in the case where a second large trianglethat is adjacent to a first large triangle determined to be a planecandidate in the plane candidate search step does not, as a whole,qualify as a plane candidate, it is determined whether three smalltriangles included in the second large triangle and adjacent to thefirst large triangle are plane candidates. Accurate estimation thattakes in the periphery of a plane can thereby be performed to theutmost.

Also, in the method for estimating a plane in a range image of thepresent invention, in a case where a plurality of regions are determinedto be a plane in the plane determination step, the largest of theregions may be taken as a floor surface. Alternatively, in a case wherea plurality of regions are determined to be a plane in the planedetermination step, which of the regions is a floor surface may beselectable by an external operation.

According to the method for estimating a plane in a range image havingsuch a configuration, a floor surface can be accurately detectedaccording to the installation environment, intended use, and the like.

Also, in the method for estimating a plane in a range image of thepresent invention, a range image serving as a camera coordinate systemmay be converted to a coordinate system in which the origin of theZ-axis is the floor surface, based on an equation of the plane taken orselected as the floor surface.

According to the method for estimating a plane in a range image havingsuch a configuration, more accurate recognition can be performed,because it is easy to find out the height or the like of a detectedhuman body or object from the floor surface.

Further, a range image camera of the present invention is provided withan imaging element capable of acquiring range information for pixelvalues and generating a range image, and an image processing unit thatperforms image processing on the range image, and in the imageprocessing by the image processing unit, any of the above configurationsof the method for estimating a plane in a range image is executed.

According to the range image camera having such a configuration, thefoot of a human body can be clearly separated from a floor surface witha small amount of computation, even in the case where, for example, thecontact point between the foot of the human body and the floor surfaceis included in the captured range image, detection of a human body,object or the like can be more accurately performed.

Because the method for estimating a plane in a range image of thepresent invention and the range image camera of the present inventionenable the foot of a human body to be clearly separated from a floorsurface with a small amount of computation, even in the case where, forexample, the contact point between the foot of the human body and thefloor surface is included in the captured range image, detection of ahuman body, object or the like can be more accurately performed.Further, falsely recognizing a smooth curve or the like as a plane canbe prevented to the utmost, and even if a very small step exists in asurface that can as a whole be taken as a plane, the surface can beaccurately recognized as being a plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a schematic configuration of a rangeimage camera 10 according to an embodiment of the present invention.

FIG. 2 shows an example of a range image displayed as 3D information inthe case where a human body standing in front of a wall with arms openhas been captured by this range image camera 10.

FIG. 3 is an explanatory diagram of a 3D data structure acquired by animage sensor 11 built into this range image camera 10.

FIG. 4 is an explanatory diagram of a cross product.

FIG. 5 is a flowchart showing a schematic of a method for estimating aplane in a range image.

FIG. 6( a) is an explanatory diagram of an operation for averaging 3Ddata every two adjacent pixels in each of rows arranged in an Xdirection. FIG. 6( b) is an explanatory diagram of 3D data converted toa houndstooth arrangement.

FIG. 7( a) is an explanatory diagram of a different operation foraveraging 3D data every two adjacent pixels in each of rows arranged inan X direction. FIG. 7( b) is an explanatory diagram of converted 3Ddata.

FIG. 8 is an explanatory diagram of deciding a triangle of interest inthe case where conversion to 3D data having a houndstooth arrangement isnot performed.

FIG. 9 is an explanatory diagram of processing that includes performinga plane candidate search while focusing on a small triangle T_(S1).

FIG. 10 is an explanatory diagram of repeating processing that includesperforming a plane candidate search while focusing on a large triangleT_(L1).

FIG. 11 is an explanatory diagram of the case where a triangle ofinterest in the plane candidate search is further enlarged.

FIG. 12 is an explanatory diagram of operations and effects resultingfrom the embodiment.

FIG. 13 is an explanatory diagram of different operations and effectsresulting from the embodiment.

FIG. 14 is an explanatory diagram of processing for addressing candidateomission in the plane candidate search.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of a rangeimage camera 10 according to an embodiment of the present invention.

As shown in FIG. 1, the range image camera 10 is provided with an imagesensor 11 (e.g., TOF sensor) capable of generating a range image afteracquiring range data for pixel values based on the time taken for lightprojected toward a target space to be reflected back, an imageprocessing unit 12 that performs plane estimation (detailed below) ofplanes in this range image, and a control unit 13 (e.g., CPU) thatperforms overall control of the range image camera 10.

FIG. 2 shows an example of a range image displayed as 3D information inthe case where a human body standing in front of a wall with arms openhas been captured by this range image camera 10.

For example, in an image formed by visible light captured with a normalimage sensor in a dark room or the like, a human body may be hardlydistinguishable from the background. In contrast, in a range imagecaptured by the range image camera 10, the surface of the human bodystanding in front of a wall with arms open emerges from the background,as shown in FIG. 2.

FIG. 3 is an explanatory diagram of a three-dimensional (3D) datastructure acquired by the image sensor 11 built into this range imagecamera 10.

This image sensor 11 acquires range information for each of a pluralityof pixels arranged in square lattice, and stores the range informationin a two-dimensional (2D) array (X, Y), which covers viewing angles thatinclude the object, as 3D data P_(XY)=(x_(XY), y_(XY), z_(XY))referenced on the position of the range image camera 10 or anarbitrarily set origin, where X is the horizontal direction and Y is thevertical direction, as shown in FIG. 3.

FIG. 4 is an explanatory diagram of a cross product. Since crossproducts are frequently used in the below-mentioned method forestimating a plane in a range image, a simple description will be givenhere for the record.

As shown in FIG. 4, the cross product a×b of space vectors a and b isperpendicular to both the space vectors a and b, and a magnitude of thecross product a×b is equal to an area S of a parallelogram spanned bythe space vectors a and b. Also, a, b and a×b form a right-handed systemin the stated order. However, a×b=0 with regard to a and b when theparallelogram is collapsed such that S=0.

Here, a×b=(a₂b₃−a₃b₂, a₃b₁−a₁b₃, a₁b₂−a₂b₁) when the space vectorsa=(a₁, a₂, a₃) and b=(a₁, a₂, a₃).

Note that given that the cross product a×b is perpendicular to both ofthe space vectors a and b, the cross product a×b is also the normalvector of a plane defined by the three vertices of a triangle, twoadjacent sides of which are the space vectors a and b.

Method for Estimating a Plane in a Range Image

FIG. 5 is a flowchart showing a schematic of a method for estimating aplane in a range image. Hereinafter, the method for estimating a planein a range image acquired by the range image camera 10 will be describedwith reference to FIG. 5. Note that because steps S7 to S9 in FIG. 5need not necessary be performed, these steps will be described below inthe section titled Variations and Other Matters.

3D data acquired by the image sensor 11 is used to estimate a plane in arange image performed as a result of the following processing by theimage processing unit 12. A “plane” as referred to here denotes a regionsuch as a floor or a wall that has a large area greater than or equal toa fixed value and that is to be separated from a range image as thebackground of a human body or the like.

(1) Data Conversion

As mentioned above with reference to FIG. 3, the actual pixels of theimage sensor 11 are arranged in a square lattice. In view of this,firstly, as shown in FIG. 6( a), an operation for averaging 3D dataevery two adjacent pixels in each of rows arranged in the X direction(horizontal direction similar to FIG. 3, with this also being the casein the following description) is performed. At this time, the pairingsof adjacent pixels to be averaged are displaced by one pixel invertically adjacent rows. 3D data converted to a houndstooth arrangement(converted 3D data) is thereby obtained, as shown in FIG. 6( b) (stepS1).

Note that while the pixel spacing of the converted 3D data in the Xdirection is actually twice that of the original spacing, as a result ofaveraging the pre-conversion 3D data every two adjacent pixels in the Xdirection, the converted 3D data is shown as having the same pixelspacing for simplicity of description (this is also the case in FIG. 9onward).

Conversion to 3D data having a houndstooth arrangement can, however, beperformed while maintaining the same pixel spacing in the X direction asthe pre-compression 3D data by employing devices such as incorporatingan interpolation operation. For example, converted 3D data such as shownin FIG. 7( b) is obtained by performing an operation (different to thatof FIG. 6( a)) for averaging 3D data every two adjacent pixels in eachof rows arranged in the X direction, such as shown in FIG. 7( a).Alternatively, an operation may be performed after setting a triangle ofinterest, such as will be discussed later, in combination with adjacentpixels such as shown in FIG. 8, without performing conversion to 3D datahaving a houndstooth arrangement.

Also, rather than performing averaging using only a single frame of arange image at one point in time, averaging may, for example, beperformed using a plurality of frames of range images on the time axis.

(2) Operation for Targeting a Triangle of Interest

Next, one of small triangles constituting the smallest unit of thehoundstooth arrangement is focused on. Here, when a small triangleT_(S1), as shown in FIG. 9, for example, is focused on, an equation of aplane defined by the vertices A1, A2 and A3 of the small triangleT_(S1), and the cross product of two vectors respectively correspondingto two sides (A1A2, A1A3) of the small triangle T_(S1) as a normalvector of this plane are computed, based on converted 3D datacorresponding to the three vertices A1, A2 and A3 of the small triangleT_(S1) (step S2).

(3) Operation Targeting Adjacent Triangles

Further, the normal vectors of the three triangles T_(S2), T_(S3) andT_(S4) (each being the same size as the small triangle T_(S1) in termsof pixels of converted 3D data) that are adjacent in such a manner as toeach share a different one of the three sides (A1A2, AlA3, A2A3) of thesmall triangle of interest T_(S1) are similarly computed.

(4) Plane Candidate Search

After deriving the cross products of all pairings (6 in total) of thenormal vectors of the four small triangles T_(S1), T_(S2), T_(S3) andT_(S4) (step S3), a size comparison is performed between the largestvalue of magnitudes of these cross products and a predeterminedthreshold (first threshold). Note that this first threshold is notlimited to a fixed value, and may be changeable according to size interms of pixels of converted 3D data of the small triangle of interestT_(S1). In this case, when the triangle of interest is enlarged asdescribed above, the first threshold can also be increased in responseto the enlarging ratio.

When the size comparison result indicates that the largest value of themagnitudes of the cross products is less than or equal to the firstthreshold, a single large triangle (triangle having the vertices A4, A5and A6) composed of the four small triangles T_(S1), T_(S2), T_(S3) andT_(S4) is determined to be a plane candidate. In other words, if thesmall triangles having the houndstooth arrangement are all the sameshape, the areas of the small triangles will all be the same, and thenormal vectors derived from the cross products will also be equal inmagnitude, which means that the difference in magnitude of the crossproducts of two normal vectors is only dependant on the angle formed bythese two normal vectors. If the largest value of the magnitudes of thecross products is less than or equal to the first threshold, thisindicates that the angle formed by the arbitrary pairing of two normalvectors is less than or equal to the angle corresponding to apredetermined value.

Similar processing is repeated after shifting the small triangle ofinterest, in order to search for the next plane candidate. For example,a small triangle T_(S5) to the upper right of the small triangle T_(S1)may be focused on. The large triangle centered on this small triangleT_(S5) will thereby be exactly adjacent to and vertically symmetricalwith the large triangle composed of the small triangles T_(S1), T_(S2).T_(S3) and T_(S4).

Repeating similar processing for all of the converted 3D data whileshifting the small triangle of interest enables the converted 3D data tobe searched for all plane candidates existing therein (step 4).

(5) Plane Determination

If the large triangle (triangle having the vertices A4, A5 and A6) isdetermined to be a plane candidate in the plane candidate search of (4)above, the large triangle T_(L1) is then focused on, and processingsimilar to (2) to (4) above is repeated using the adjacent three largetriangles T_(L2), T_(L3) and T_(L4), as shown in FIG. 10. At this time,these large triangles T_(L1), T_(L2), T_(L3) and T_(L4) are taken assmall triangles T_(S1), T_(S2), T_(S3) and T_(S4).

The triangle of interest in the plane candidate search is furtherenlarged, as shown in FIG. 11. That is, a triangle T_(X1) is focused on,and similar processing is subsequently recursively repeated (step 5). Inthe case where, however, a triangle that has been ruled out as a planecandidate is included in the enlarged triangle of interest T_(X1), thistriangle T_(X1) is ruled out as a plane candidate.

If, as a result of recursively repeating the plane candidate searchprocessing in such a manner, the area of a new large triangle that isdetermined to be a plane candidate is greater than or equal to apredetermined area (second threshold), the region corresponding to thatlarge triangle is determined to be a plane. The number of repetitions ofthe processing preferably is set with a prescribed number of repetitionsas the upper limit (step 6).

A plane can thus be efficiently estimated by repeating recursiveprocessing while enlarging the triangle of interest.

(6) Separation of Plane from Range Image

All of the regions that are determined to be a plane as a result of theprocessing from (1) to (5) above are separated from the range image. Forexample, the regions determined to be a plane may be redrawn on theplane without using the captured range image. Because this enablespixels corresponding to a plane such as a floor or a wall in the rangeimage to be handled separately from the other pixels, detection of ahuman body, object or the like can be accurately performed.

When applied to a smooth curve F1, as shown in FIG. 12, the angle willbe extremely shallow in the case of only a small triangle composed of apoint P_(n) and points P_(n+1) and P_(n−1) adjacent thereto, which maylead to the curve being falsely recognized as a plane. On the otherhand, the method for estimating a plane in a range image using theabovementioned recursive processing enables such false recognition to beprevented, since the angle will be sharper in the case of a largetriangle composed of the point P_(n) and points P_(n+4) and P_(n−4) thatare slightly removed therefrom.

Also, in the case where a very small step B2 exists in a face F2 thatcan as a whole be taken as a plane, as shown in FIG. 13, it can beanticipated that calculation of the cross product will result in a smallvalue, because the area of the triangle at the step B2 at this time willbe small. Thereby, even in the case of having the small step B2, theface F2 that includes this step B2 can be accurately recognized as aplane.

Variations and Other Matters

For example, in the plane candidate search of (4) above, even if a largetriangle composed of the small triangle of interest T_(S1) and the threeadjacent small triangles T_(S2), T_(S3) and T_(S4) is not determined tobe a plane candidate, a large triangle T_(L1) adjacent to this largetriangle could possibly be determined to be a plane candidate, as shownin FIG. 14.

In view of this, as plane candidate omission processing in such a case(step 7), the small triangles T_(S2), T_(S3) and T_(S4) may each betaken as a plane candidate if the magnitude of the cross product of thenormal vector of the large triangle T_(L1) and the normal vector of eachof the small triangles T_(S2), T_(S3) and T_(S4) is less than or equalto a predetermined threshold. A subsequent plane candidate search may,however, be performed using the large triangle composed of the smalltriangles T_(S1), T_(S2), T_(S3) and T_(S4). Such plane candidateomission processing is recursively performed while reducing the size ofthe triangle of interest (step 8), and the number of repetitions of theprocessing preferably is set with a prescribed number of repetitions asan upper limit (step 9).

Also, in the case where a plurality of regions have been determined tobe planes in the plane judgment of (5) above, the largest region may beautomatically taken as the floor. Alternatively, the plurality ofregions may be displayed on a monitor or the like, and one of theregions may be selected as the floor as a result of an externaloperation by a user using a mouse, keyboard or the like.

Further, a configuration may be adopted in which the installation siteof the range image camera 10 in space, the angle of the imagingdirection, and the like are calculated based on the equation of a planerepresenting the floor determined in this manner, and a range imageserving as the camera coordinate system is converted to a coordinatesystem in which the floor is the origin of the Z axis.

The present invention may be embodied in other forms without departingfrom the gist or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allmodifications and changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1. A method for estimating a plane in a range image, comprising: a dataconversion step of computing converted three-dimensional data that isconverted to a houndstooth arrangement, based on three-dimensional dataof pixels arranged in a square lattice; a triangle-of-interestcomputation step of focusing on one of small triangles constituting asmallest unit of the houndstooth arrangement, and computing, based onconverted three-dimensional data corresponding to three vertices of thesmall triangle of interest, an equation of a plane defined by the threevertices, and a cross product of two vectors corresponding respectivelyto two sides of the small triangle of interest as a normal vector of theplane; an adjacent triangle computation step of computing a normalvector for each of three small triangles that are adjacent in such amanner as to respectively share a different one of three sides of thesmall triangle of interest; a plane candidate search step of searchingfor a plane candidate by performing, for all of the convertedthree-dimensional data while shifting the small triangle focused on inthe triangle-of-interest computation step, a process of deriving alargest value among respective magnitudes of cross products of allpairings of the normal vector computed in the triangle-of-interestcomputation step and the three normal vectors computed in the adjacenttriangle computation step, and a process of determining a large trianglecomposed of the small triangle of interest and the three adjacent smalltriangles to be a plane candidate if the largest value is less than orequal to a first threshold; a plane determination step of sequentiallyrepeating the triangle-of-interest computation step, the adjacenttriangle computation step and the plane candidate search step with alarge triangle determined to be a plane candidate in the plane candidatesearch step taken as the small triangle of interest in thetriangle-of-interest computation step, recursively performing therepetition while enlarging the small triangle of interest, and, when anarea of a new large triangle determined to be a plane candidate isgreater than or equal to a second threshold, determining a regioncorresponding to the large triangle to be a plane; and aplane-determined region separation step of separating the regiondetermined to be a plane in the plane determination step from a rangeimage.
 2. The method for estimating a plane in a range image accordingto claim 1, wherein even in a case where a second large triangle that isadjacent in such a manner as to share one side of a first large triangledetermined to be a plane candidate in the plane candidate search step isnot determined to be a plane candidate, if a magnitude of a crossproduct of a normal vector of the first large triangle and a normalvector of each of three small triangles included in the second largetriangle and adjacent to the first large triangle is less than or equalto a third threshold, the small triangles are each taken as a planecandidate.
 3. The method for estimating a plane in a range imageaccording to claim 1, wherein in a case where a plurality of regions aredetermined to be a plane in the plane determination step, a largest ofthe regions is taken as a floor surface.
 4. The method for estimating aplane in a range image according to claim 1, wherein in a case where aplurality of regions are determined to be a plane in the planedetermination step, which of the regions is a floor surface isselectable by an external operation.
 5. The method for estimating aplane in a range image according to claim 3, wherein, based on anequation of the plane taken or selected as the floor surface, a rangeimage serving as a camera coordinate system is converted to a coordinatesystem in which an origin of a Z-axis is the floor surface.
 6. A rangeimage camera comprising: an imaging element capable of acquiring rangeinformation for pixel values and generating a range image; and an imageprocessing unit that performs image processing on the range image,wherein in the image processing by the image processing unit, the methodfor estimating a plane in a range image according to claim 1 isexecuted.
 7. The method for estimating a plane in a range imageaccording to claim 2, wherein in a case where a plurality of regions aredetermined to be a plane in the plane determination step, a largest ofthe regions is taken as a floor surface.
 8. The method for estimating aplane in a range image according to claim 2, wherein in a case where aplurality of regions are determined to be a plane in the planedetermination step, which of the regions is a floor surface isselectable by an external operation.
 9. The method for estimating aplane in a range image according to claim 4, wherein, based on anequation of the plane taken or selected as the floor surface, a rangeimage serving as a camera coordinate system is converted to a coordinatesystem in which an origin of a Z-axis is the floor surface.
 10. Themethod for estimating a plane in a range image according to claim 8,wherein, based on an equation of the plane taken or selected as thefloor surface, a range image serving as a camera coordinate system isconverted to a coordinate system in which an origin of a Z-axis is thefloor surface.
 11. A range image camera comprising: an imaging elementcapable of acquiring range information for pixel values and generating arange image; and an image processing unit that performs image processingon the range image, wherein in the image processing by the imageprocessing unit, the method for estimating a plane in a range imageaccording to claim 2 is executed.
 12. A range image camera comprising:an imaging element capable of acquiring range information for pixelvalues and generating a range image; and an image processing unit thatperforms image processing on the range image, wherein in the imageprocessing by the image processing unit, the method for estimating aplane in a range image according to claim 3 is executed.
 13. A rangeimage camera comprising: an imaging element capable of acquiring rangeinformation for pixel values and generating a range image; and an imageprocessing unit that performs image processing on the range image,wherein in the image processing by the image processing unit, the methodfor estimating a plane in a range image according to claim 4 isexecuted.
 14. A range image camera comprising: an imaging elementcapable of acquiring range information for pixel values and generating arange image; and an image processing unit that performs image processingon the range image, wherein in the image processing by the imageprocessing unit, the method for estimating a plane in a range imageaccording to claim 5 is executed.
 15. A range image camera comprising:an imaging element capable of acquiring range information for pixelvalues and generating a range image; and an image processing unit thatperforms image processing on the range image, wherein in the imageprocessing by the image processing unit, the method for estimating aplane in a range image according to claim 7 is executed.
 16. A rangeimage camera comprising: an imaging element capable of acquiring rangeinformation for pixel values and generating a range image; and an imageprocessing unit that performs image processing on the range image,wherein in the image processing by the image processing unit, the methodfor estimating a plane in a range image according to claim 8 isexecuted.
 17. A range image camera comprising: an imaging elementcapable of acquiring range information for pixel values and generating arange image; and an image processing unit that performs image processingon the range image, wherein in the image processing by the imageprocessing unit, the method for estimating a plane in a range imageaccording to claim 9 is executed.
 18. A range image camera comprising:an imaging element capable of acquiring range information for pixelvalues and generating a range image; and an image processing unit thatperforms image processing on the range image, wherein in the imageprocessing by the image processing unit, the method for estimating aplane in a range image according to claim 10 is executed.